The present invention relates generally to propulsion systems for aircraft or space vehicles such as airplanes, aerospace vehicles, rockets and other spacecraft.
On-board propulsion systems such as air breathing engines and rocket propulsion devices have been devised for accelerating vehicles to high speed. Rocket propulsion systems for launching vehicles into space are often chemically powered and contain one or more internal propellants. These systems have two major limitations. One is that the duration of thrust is limited by the amount of propellant the rocket can contain. Another is that only a fraction of the propellant energy goes into the payload. Much of the propellant energy goes into accelerating and lifting the mass of propellant and propulsion related components, leaving little room for the non-propulsive elements of the vehicle and the payload. With the current state of the art of rocket flight from earth to space, approximately 90% of the initial weight is propellant related, 5% is structural, and only 5% can be allotted to the payload to be transported into space. This is clearly an inefficient system.
Air breathing jet engines produce thrust in the same manner as rocket engines, but draws one of its propellants from the ambient gas through which it flies, requiring only about 1/8 of the propellant load of an equivalent rocket engine that burns hydrogen and oxygen. A disadvantage of these engines is that oxygen availability decreases with altitude, and that the vehicle must fly at a higher altitude with increasing speed in order to keep atmospheric drag less than the thrust. Thus, design of an air breathing earth to orbit vehicle, or aerospace plane, which would operate safely and efficiently would be extremely difficult. With increasing Mach number, it becomes progressively more difficult to inject fuel into and through the boundary layer, and the rapidly moving air stream may sweep fuel out of the engine before chemical burning is complete. Additionally, the effectiveness of aerodynamic lift decreases with increasing Mach number, requiring use of a greater fraction of the thrust to drive the craft upward. For these reasons, the aerospace plane must fly a very exact trajectory at low acceleration, which results in prolonged heating and loss of much energy to drag. The types of materials needed for construction of an aerospace plane to withstand the prolonged heating are not available at this time. Thus, there are still many outstanding problems with existing aerospace plane proposals, and such vehicles would still have to carry a significant weight of propellant, reducing payload capacity.
Another recent concept in attempting to alleviate the problems of internal propellant load is that of the ram accelerator. In this proposal, a long tube is filled with a combustible mixture of gases such as oxygen and methane or hydrogen. A cylindrically symmetric vehicle in the form of the compressor section of a ram jet is introduced into the tube at sufficient speed to compress the ambient gas to form a shock about aft of its mid-body. Combustion of the gas occurs aft of the shock. The vehicle does not have to carry any propellant. Exit speeds in excess of 10 km. per second can be obtained. A quick opening door or breakable barrier at the end of the tube permits the vehicle to exit. However, this system has a number of limitations, and does present hazards. It is doubtful that a ram accelerator could be used to launch people into space or between points on earth due to the accelerations in the tube, and also due to atmospheric deceleration of the vehicle on leaving the tube. Also, a tube several hundred feet long that is repeatedly filled with a combustible mixture of gases presents an intrinsic hazard. All or a major portion of the capital investment can be lost due to one explosion or due to a vehicle which hits the tube at high speed. A given tube can eject vehicles into only a limited set of trajectories. Little adjustment can be made to the vehicle dimensions to accommodate different payloads, since a vehicle which is too long or too heavy cannot be adequately accelerated by a given tube and gas mixture.
Thus, existing and currently proposed propulsion systems for earth to space or flights from one region of earth to another do not overcome the problems of devoting a major portion of the flight energy to the transportation and acceleration of propellant mass, considerably reducing payload capabilities.